[Live-devel] Fragmented intra frames

[Ross Finlayson]

> I have never quite got the advantage of frame fragmentation, i.e. let's consider a large intra frame and the two cases:
> 
> 1) Sending the intra frame as a single huge packet
> 
> 2) Sending the intra frame as a series of smaller packets

Your use of the word ‘packet’ here is strange (and, I presume, wrong), because the word ‘packet’ usually means an atomic unit of data sent over a network.  I presume, instead, that you meant to ask about the difference between:
	1) Sending an intra frame (i.e., ‘key frame’) as a single H.264 NAL unit, versus
	2) Sending an intra frame (i.e., ‘key frame’) as a sequence of H.264 ‘slice’ NAL units
In each case, each NAL unit (unless it’s very small) will be fragmented into several network (i.e., RTP/UDP/IP) packets - which can be up to 64 kBytes in size, but are typically about 1500 bytes.  The loss of *even one* of these packets will make the whole NAL unit useless (i.e., unrenderable) for the receiver.

> The behaviour for the two cases in the case of packet loss:
> 
> 1) The whole intra frame is lost, must wait for the next one

Correct

> 2) A one slice of the intra frame is lost .. making the whole intra-frame useless.

Incorrect.  The video decoder/renderer will be able to decode/render/display the other slices of the frame - apart from the one that was lost.  This might look strange, but at least you’ll be able to see something.


Ross Finlayson
Live Networks, Inc.
http://www.live555.com/